Divalent metal salts such as metal sulfates have been widely employed as supplemental sources of trace metals in agronomy and animal nutrition. However, the high reactivity of metal sulfates can be disadvantageous when the salts are added directly to animal feeds or to vitamin-containing feed premixes. For example, scavenger compounds such as phytic acid and phosphates can form biologically-unavailable complexes with free metal ions in the stomach of animals and in the rhyzosphere of plants. To compensate for these reactions, high levels of metal sulfates have been added to fertilizers and feed formulations. However, the overuse of trace minerals can also give rise to nutritional problems, e.g. due to metal-metal interactions in vivo, and can contribute to environmental pollution. For example, see N. T. Davies, "Antinutrient Factors Affecting Mineral Utilization," in Proc. Nutr. Soc., 38, 121 (1979). Furthermore, metal sulfates can adversely effect the activity of other feed components such as vitamins.
Attempts to overcome these disadvantages have led to the introduction of metal salts into feeds and fertilizers in combination with various chelating or sequestering agents. Chelating agents are recognized as those in which the metal ion-substrate bonds are substantially covalent in nature. Complexes of metal salts with proteinates, gluconates and EDTA involve bonds of this type. However, the high strength of the metal-substrate bonds often reduces the ultimate bioavailability of the metal ion.
Sequestered metal ions, on the other hand, are generally more bioavailable because the sequestering interactions usually do not involve the formation of highly covalent bonds. Instead, the metal ions are physically entrapped by the substrate matrix and retained by weaker bonding which is primarily electrostatic in nature. Selection of a sequestrant which can be readily broken down by enzymes permits the effective in vivo absorption of the metal ions.
For example, U.S. Pat. Nos. 3,764,341 and 3,876,810 are directed to metal sulfates sequestered via the polysaccharide components of sea kelp. However, the water-insoluble nature of the fibrous portion of the kelp renders the product unsuitable for use in liquid feeds such as milk replacers and for agricultural applications requiring spraying.
Therefore, a need exists for a feed or fertilizer composition including sequestered metal ions, which is also highly water dispersable and/or water soluble. A further need exists for a nutrient composition which can stabilize metal ions against undesirable interactions with other components of the composition or with chemicals present in the target species, while preserving the bioavailability of the metal ions.